Many systems, such as television receivers, video cassette recorders, audio signal processors, etc., generate control signals to condition the systems to operate reliably for input signals occurring within a range of amplitude or frequency parameters. An example of such control signals in a television receiver include automatic gain control (AGC) signals and automatic chrominance control (ACC) signals. These control signals are generally generated in a closed loop subsystem including for example a gain controlling element having an input terminal for receiving a signal to be controlled and an output terminal at which parametrically controlled input signal is available. Control signals are generated by apparatus which is responsive to the controlled output signal, for measuring a particular parameter of the signal against a reference value, and, producing, e.g., a difference value for controlling the control element.
In many systems it is undesirable for abrupt changes in control signals to be applied immediately to the signal controlling element, because abrupt signal changes may produce adverse affects. For example, abrupt changes in AGC signals may produce flicker in a reproduced video image. Therefore, control signals are frequently coupled to the signal controlling element via a low pass filter.
Currently many of the aforementioned systems are being implemented with digital hardware wherein signals are processed in binary signal format. (Herein "binary signal" is intended to mean sampled data signals formed of ones and zeroes defining numbers representative of a parameter (e.g., amplitude) of a signal, and is meant to include binary signals, two's complement signals, BCD signals, grey code signals, etc.) When processing signals in binary format (hereinafter digital format) it may be inconvenient for various reasons to realize certain processing functions with circuitry which is a simple digital analogue of an analog circuit. An example is a digital low pass filter having a long time constant which may require many sample delay elements, weighting circuits and digital summing circuits.